Because you let him as a puppy most likely..some digs are great watch dogs. I have a Boston terrier and she let's us know when anybody pulls in our driveway..but stops as soon as I tell her..watch the dog whisperer..seriously you will learn alot
Curiousity, territorial, or just plain annoying. They learn it growing up if bad habits aren't discouraged and good habits reinforced. It's all how you want to raise your dog. Now since there older it's harder but just work with them and it'll learn.